Vehicles providing an elevatable work surface, and particularly truck or truck-like vehicles capable of unrestricted operation over the public highways and providing such a work surface, find considerable utility in providing work stations for servicing public utility facilities, highway signs and signals, lights and the like as well as in constructing buildings, piping systems and the like. These vehicles generally can be divided into two general groupings depending upon the nature of the work surface provided.
In one grouping, the work surface is a bucket-like work surface generally capable of containing 1 or 2 men and a relatively limited quantity of tools, parts for installation and the like. Traditionally these buckets have been mounted upon a variety of vehicles in a variety of manners, but more recently have generally come to be mounted upon an elevatable boom, generally articulated, but from time to time of a telescoping nature, affixed to the chassis of a medium or heavy duty conventional truck. Generally, the booms are mounted to a pivotable chassis mounted turret rotatable throughout 360.degree., and provide a work station moveable both vertically and horizontally to a variety of heights and to a number of distances from the truck.
However, where a task to be performed requires that more than two men be elevated at a time, or where bulky, or a large quantity of materials are required at the elevated work site, these bucket-like work surfaces find some limitations. Additionally, since the bucket necessarily hangs away from the truck during most work functions, should the vehicle experience a failure limiting or eliminating motion of the bucket, egress for occupants of the bucket can be most difficult. Often, therefore, a two man crew is required for operating such vehicles, one remaining upon the ground in the event of mobility loss, even though the task occupying the vehicle requires only one man for safe completion. Often these telescoping boon trucks, partially due to the angle of the attachment between boom and turret, require a relatively low body profile. Enclosed storage space aboard the vehicle is thereby somewhat limited.
The other grouping of vehicles includes trucks having generally platforms elevated only vertically from the truck, reaching a work site positioned relatively directly above the truck. These platforms generally provide a relatively flat, spacious working area often including guard railings and/or other protective measures for restraining men working upon the platform from falling overboard. Often these platforms are considerably longer than wide and are oriented longitudinally with the truck during transportation along public highways. In some proposals these platforms have been partially rotatable when elevated for gaining some access to work located slightly to one side or the other of the truck. In one proposal the platform includes an extension slidingly received in the platform that may be used for extending the platform to reach work more remote from the truck.
With these platform service vehicles, at least two general limitations have, from time to time, performed to reduce their utility. One such limitation relates to stability. The platform being generally large and heavy when compared to the buckets used on other vehicles, a large force moment can be generated by the platform when fully elevated, making the work surface as carried upon the truck possibly unstable and therefore potentially unsafe. One proposal for stabilizing the work surface has been to mount the platform upon 4 tubular masts mounted in a generally rectangular pattern upon the truck each including a telescoping cylinder for elevating the platform and tubular mast. One difficulty with this proposal appears to have been to make mechanisms both for raising and for rotating the platform somewhat bulky and heavy, which can correspondingly reduce the load bearing capability of the platform. Another drawback with this proposal appears to have been to make such a vehicle less competitive from a cost aspect, the vehicle requiring 4 hydraulic lift cylinders and related equipment, 4 masts, and the like.
In another proposal, a single mast has been utilized, comprised of nested opposing channel members each set of opposing channel members being received outside of and being attached to parallel frame members of the truck chassis. With the platform fully extended, however, these nested channel members have tended to spread from an optimal alignment and todistort under torque applied by the elevated tower causing some difficulty in operation, particularly, where lower portions of such nested channel members comprising a mast for such a platform vehicle have not been securely joined by crossmembers.
In another general limitation, the extent of rotational mobility of the platform can impact upon its utility. In prior proposals, the platform has not been rotatable through 360.degree. in either direction, that is clockwise or counterclockwise, occasionally necessitating turning the tower almost completely about simply to gain a small change in the rotational orientation of the platform. Such platforms were not freely rotatable, that is they could not be rotated to and locked into an infinite number of rotational positions, being generally restricted to being positively locked at about one or two dozen positions within the span of platform rotation. Additionally, such platforms generally were hand cranked for rotation, potentially a quite difficult undertaking where the truck is on other than a level surface.
In another general limitation the extent of electrical insulation of the work surface from the truck, and therefore the relative safety of the work surface near electrical wiring, has been limited to about 300 volts. While to some extent, the insulative capability of the work surface is related to the nature of insulators used to separate the work surface from the mast(s) and truck, in part it is related to the physical configuration of the platform and its interconnection to the mast(s).
In still another general limitation, platforms, while generally being raised under power, usually hydraulic, have been allowed to lower under the impetus of gravity. Gravity lowering, where telescoped mast members are slidingly interacting, can trigger an uneven lowering causing cocking and jamming of the mast mechanism. These difficulties can be aggrevated where the mast frame emebers have become distorted due to torque loading attributable to the presence of the platform atop the mast.
In still another general limitation, telescoping sections of the mast(s) have not been specifically tied to a particular section of a hydraulic cylinder utilized for raising the platform. As a result, from time to time less structurally efficient sections of a mast may have been telescoped preferentially due to binding between the sliding mast sections.
A platform service vehicle capable or self propelled transport over public highways, having a single telescoping mast bearing a platform that can be horizontally rotated through 360.degree. either clockwise or counterclockwise and that can can be locked into an infinite number of rotational positions would find considerable utility. Where, further such a platform service truck can provide a work platform having an electrical insulating capability of 5000 volts or more as is often necessary to work upon modern electrical facilities, and where such a truck provides exceptionally smooth, reliable operation of the telescoping mast and rotating platform utilizing but a single motive power source, its utility could be substantial.